The present invention relates to an article that combines by, for example, soldering or brazing, a first material and at least one additional material, at least a portion of which comprises a sacrificial constraint. Preferably, these two materials have substantially different coefficients of thermal expansion (CTE). More preferably, the first material includes at least one cemented carbide and the at least one additional material having a substantially different CTE than the cemented carbide functions as the sacrificial constraint. The sacrificial constraint facilitates the fabrication of the article with substantially fewer quality control rejections and fewer failures which can be premature to provide a longer, and more consistent useful life to the article as compared to the prior art articles designed for the same use. More particularly, the present invention relates to a cutter bit for use in conjunction with excavation equipment. Even more particularly, the invention relates to a concave cutter bit, preferably rotatable, for use in conjunction with excavation equipment such as, for example, a longwall shearer, a continuous mining machine, a trencher, a road milling machine, an auger, and a saw.
Some conventional cutter bits used for excavation equipment use a single cutting element at a forward end. In this particular application, it is only this single cutting element that forms the effective cutting element of the cutter bit that impinges upon and cuts or fractures the substrate such as, for example, earth strata. The balance of the forward end of the cutter bit pushes fractured or cut material out of the path of the cutter bit.
Another style of cutter bit for use with excavation equipment is a concave cutter bit. The typical concave cutter bit has an enlarged diameter portion, which contains a concavity, at the forward end. A cutter element of hard material such as, for example, cemented tungsten carbide, surrounds the outer periphery of the concavity so that the cutter element presents a generally circular or ring-like shape. One example of a concave cutter bit is illustrated by U.S. Pat. No. 5,078,219 to Morrell et al. Another example of such a cutter bit is shown by U.S. Pat. No. 5,333,938 to Gale.
The cutter element can take the form of a single piece ring such as is shown by Morrell et al. Typically, the cutter element is made from cobalt-cemented tungsten carbide and the bit body is made from steel. The cutter element is secured to the steel bit body by brazing so that, at a minimum, there is a braze joint between the bottom surface of the carbide cutter element and the surface of the cutter bit body.
Carbides such as cobalt-cemented tungsten carbide have coefficients of thermal expansion that are approximately one-half to one-third that of steel. This difference in thermal expansion results in contracting at different rates upon cooling after a brazing operation. This difference in contraction can create stresses in the steel bit body, the cemented tungsten carbide cutter element, or the braze joint, or any combination of the preceding. In turn these stresses can produce cracks in the steel bit body, the cemented tungsten carbide cutter element, or the braze joint, or any combination of the preceding.
Inferior quality such as the existence of cracks in the steel bit body, the cemented tungsten carbide cutter element, or the braze joint, or any combination of the preceding, can cause the bit to be discarded as scrap. The existence of cracks or brazing stresses also can eventually lead to the premature failure of the concave cutter bit during use. It is apparent that the inability of the cutter bit to either pass quality control examination or function well by failing prematurely in the field is undesirable.
The cutter elements can also take the form of a plurality of segments positioned adjacent to one another in an end-to-end relationship to form a complete ring. It has been found, however, that the presence of cracks and braze stresses are not reduced by the use of a plurality of cutter insert segments in comparison to a cutter bit with a single piece ring-shaped cutter element. For those cutter bits where the cutter element comprises a plurality of segments, each segment is positioned with its end surfaces near, but slightly spaced apart from, the corresponding end surface of the adjacent cutter element. In the past, the distance of the spacing has been about 0.5 mm (0.020 inches).
During brazing, braze alloy flows between the opposite ends of adjacent cutter element segments to form a continuous volume of braze alloy between the opposite end surfaces of the adjacent cutter element segments. A volume of braze alloy also exists between each one of the cemented tungsten carbide cutter element segments and the steel cutter bit body.
Upon initial cooling after the brazing operation, the braze joint between the opposite end surfaces of adjacent cutter element segments solidifies as does the braze joint between the cutter element segments and the cutter bit body. At this point, however, the steel cutter bit body and the cutter element segments must still cool to room temperature.
As the cutter bit and cutter element segments continue to cool and contract, the difference in the rate of contraction between the steel bit body and the cutter element segments, which now behave as if they were one piece, creates stresses in the steel bit body, braze, and the cutter element segments. The stresses can become so great that some of the cutter element segments or braze, or both, crack.
It thus becomes apparent that the problems associated with brazing stresses, braze joint cracks, and cutter element segments cracks exist for concave cutter bits having either a single piece ring-shaped cutter element or a cutter element comprising a plurality of segments where a continuous braze joint forms between the opposing end surfaces of the adjacent segments.
Thus, it would be desirable to provide an improved concave cutter bit that does not experience, or at least has reduced, stresses in the steel bit body, the cutter elements, or the braze joint, preferably a combination of the preceding. As a consequence, such a concave cutter bit would experience less quality control rejections, as well as fewer premature failures so as to provide a longer and more consistent useful life.
One approach to addressing the above described difficulties is presented in commonly owned U.S. Pat. No. 5,456,522 issued in the name of Beach relating to a concave cutter bit which has a bit body that contains a concavity in an axially forward end. A plurality of cutter inserts are brazed to the bit body at the periphery of the open end of the concavity. Cutter inserts are spaced-apart in such a fashion so that a gap exists between adjacent cutter inserts that is of sufficient size to prevent the formation of a continuous braze joint between any adjacent cutter inserts.
Another novel and different approach comprises the present invention.